A. Field
The invention relates to a method for rotating a rotatable part by use of a hydraulic piston-cylinder drive operated by a hydraulic pressure source and having at least one piston as well as a hydraulic cylinder and a ratchet, wherein a torque is applied to the rotatable part during a loading stroke and the piston is moved into a starting position via a return stroke.
B. Related Art
Methods for rotating a rotatable part are known particularly for operation of hydraulic power wrenches.
In practice, hydraulic power wrenches are operated primarily by two methods, the so-called torque method and the so-called torque/torque-angle method. Both methods include a method step in which a defined torque is applied on the part which is to be rotated.
For measuring the torque applied onto the to-be-rotated part, it is known to provide a torque sensor. Such a method is already known from WO 03/013797 A1. The provision of added torque sensors causes additional expenditure and has the consequence that only certain types of power wrenches are suited for practicing the method. Further, such sensors are arranged on the power wrench itself, thus rendering the device sensitive to contamination and environmental influences. Since power wrenches are frequently used on construction sites, this sensitivity to environmental influences and contamination is considered to be highly disadvantageous.
Further, it is known to detect pressure data of the pressure source that is feeding the piston-cylinder unit of a power wrench, and to conclude from these data on the torque applied to the part which is to be rotated. For this purpose, on the basis of the known geometry of the power wrench, the pressure that the pressure source exerts onto the piston of the piston-cylinder unit is converted into a torque.
It is known, for instance, to control such power wrenches manually in that, for the pressure source, a maximum pressure is preset which corresponds to the desired torque. In the process, the operator has to initiate the advance and return strokes manually. In such a method, however, the system cannot detect whether the pressure exerted by the pressure source is really applied as a torque on the to-be-rotated part. It may happen that the piston of the piston-cylinder unit has abutted against an end stop and the pressure source is increasing the pressure still further. For this reason, this method generally necessitated a visual check on the side of the operating personnel for detecting whether, when the preset end pressure was reached, there had beforehand occurred a rotary movement of the to-be-rotated part. Particularly in very large power wrenches wherein a very slow angular speed of the to-be-rotated part is generated, such a visual check can be performed only with considerable difficulty.
In modern power wrench systems, the power wrenches are automatically controlled by a control device wherein, as soon as the pressure corresponding to the desired torque is reached, the pressure source will be automatically switched off or a phase of angle-controlled rotation will be initiated.
In the process, the control device cannot detect a situation where the exerted pressure is being applied merely on an end stop and there does not occur a transmission of the pressure to the rotatable part. As a consequence, also such an automatic control makes it necessary for the operator to carry out a visual check.
Thus, there are known various control methods for hydraulic power wrenches wherein an automatic control of the pressure source is provided. DE 102 22 159 A1, for instance, describes a method wherein the temporal development of the pressure of the pressure source is evaluated. From the change of the gradients of the pressure development, there are obtained signals for returning the piston-cylinder unit and for terminating the process. The process of torque application will be terminated as soon as it has been verified that the desired torque has really been impressed on the fastening element. The advantage of this method resides in that no sensorics is required on the hydraulic wrench itself since the pressure signals can be measured directly in the pressure source. In this method, the problem exists that the gradient change between the gradient of the pressure during pressure build-up at the start of the stroke, i.e. at a time when no rotation of the to-be-rotated part is occurring yet, the gradient of the pressure during the rotating movement, and the gradient of the pressure after abutment of the piston on an end stop at the end of the load stroke, are massively influenced by the volume of the piston-cylinder unit and the volume of the hydraulic tube which connects the pressure source to the hydraulic wrench. Since particularly the gradient change between the gradient of the pressure during the rotating movement, and the gradient of the pressure after abutment of the piston at the end of the load stroke is of interest because it has to be detected which torque has really been impressed on the rotatable part prior to the abutment of the piston onto the end stop, this method allows for a reliable control only if the pressure development during the gradient change includes a discontinuity. Particularly in case of large tools and weak pressure sources, the pressure development of a gradient change can show a steady development so that a precise detection of the torque applied to the rotatable part prior to the abutment of the piston onto the end stop will not be possible at all or only with difficulties.
Further, even in identical systems, i.e. identical tube lengths and identical volumes of the piston-cylinder units, the gradients which are to be evaluated will be influenced by parameters of the to-be-rotated parts, so that a pressure-gradient-based control of the pressure source may happen to react in an ambiguous manner in different applications.